Tangential and not
directly relevant, but I was testing an embedded ARM system
on Tuesday specifically for SDcard write performance, and on
a Class-10 SDcard was able to sustain 40Mbyte/second on a
32Mbyte "sprint"--for 100 iterations. That's unrelated to
USRP performance,
but at least on ARM hardware writing to "decent" SD cards,
you can get pretty-good performance.

Just to add more to this discussion in case anyone is
curious:

Unfortunately, this doesn't mean that ARM-based SoCs will
follow this trend. Using the ARM architecture doesn't
guarantee good SD Card support; it really comes down to the
manufacturer of the chip itself. For the E1xx devices, we are
reliant on TI's capability to work well with SD Cards, since
it is the OMAP3 chip acting as the controller (assuming, of
course, that support in the Linux kernel is solid).